The objective of this study is to measure the effects of intravenous nutrition with different amino acid intakes on whole body protein turnover in critically ill infants and children. We hypothesize that the provision of the age determined recommended daily requirement (RDA) of protein to these critically ill children will result in an increase in rates of protein synthesis with no effects on protein breakdown. Furthermore, we hypothesize that provision of protein above the RDA will not result in any further increase in protein synthesis.